The subject matter of the present invention relates generally to forming a metal layer on a siliceous substrate, and more particularly to forming a metal layer on a siliceous substrate by first vacuum depositing a thin metal layer and then electroplating to provide the desired metal thickness.
Numerous methods have been disclosed in the prior art to provide a metalized layer on substrates having a high silica content, such as fused silica, glass, or quartz. One of the more common methods has been by vacuum deposition, which, while providing a satisfactory bonded metallic layer to the substrate, is a time consuming and expensive process. For vacuum depositing conductive metals such as gold, or even copper, it is known to first vacuum deposit a thin layer of oxide forming metal, such as chromium, tantalum, titanium, or nichrome, which reacts with the surface of the siliceous substrate to form a reliable bond therebetween. The layer of oxide-forming metal need only be on the order of 200 Angstroms in thickness. Thereafter, while still in the evacuated environment, the gold or other conductive metal is deposited thereon to the desired thickness.
In the fabrication of hybrid circuits, the above-outlined procedure has been utilized to provide a metalized layer on siliceous substrates to form the basis for metal conductors and pads. An etching process is utilized to remove the undesired portions of the metalized layer, leaving only the conductors and pads. However, the etching is difficult to control, particularly in microcircuit applications where close tolerances are necessary. Additionally, if a precious metal such as gold is used for the metalized layer, an additional process is required to recover the removed metal from the etchant.
It has long been recognized by investigators in the art that it would be desirable to vacuum deposit on a siliceous substrate a thin metal layer, and then electroplate the metal layer to the desired thickness. Not only is such a process more simple and inexpensive, but extremely close tolerances of conductors and pads may be maintained. However, attempts to carry out this process have resulted in a loss of adhesion between the vacuum deposited metal and substrate during the electroplating process.